Obviously an eight wheeled machine has better flotation on soft ground but is not as easy to manuever in tight places. As far as a big difference, there is probably not a large difference.
I have an six wheeled Valmet 840 with tracks and it has taken me into some soft spots with big loads with few problems. However you can always get into some difficult situations when you least expect it. Just yesterday I went down the road to pull another six wheeled machine out that was stuck. The problem was they had driven over a dead tree in the path and the back bogies which were carrying a big load could not climb over the log and the front had found a soft spot and went down quick. those kind of things can happen with either machine. The guy, actually it was a young lady, probably could have gotten thru the spot if they had their tracks on, but they were laying at the landing. I never run without the tracks, winter or summer.
Ground disturbance will be worse with the tracked processor, and you can cause some limited rutting with the single wheels on the six wheeled machine. But again tracks on the bogeys will limit the rutting with the forwarder.
And the eight wheeled machines are supposed to ride better over rough ground.